Generally it is required for an exchange system not to disconnect a speech path of a pre-engaged call in any case. However the exchange system may be restarted in its entirety after the occurrence of a system failure due to its greatly complex mechanism comprised of hardware and software. In such a case, there may be a problem that a subscriber's speech path engaged in a preceding telephone call is disconnected because both the software and the hardware have to be initialized when restarting the system.